hearing

The mechanism of hearing in the ear. In these two
illustrations above, one natural a diagrammatic, the dashed red lines
show the route along which sound vibrations are from the external
auditory meatus (ear canal) to the organ of Corti in the cochlea.

Sound consists of waves of compression
and rarefaction in the air. The intensity of the sound depends on the size
of the waves, and the pitch depends
on their frequency, or closeness together.

When sound waves reach the ear they pass down
the ear canal (auditory canal) until they
arrive at the eardrum. The sound waves vibrate
the eardrum and the vibrations are transmitted to the handle of the small
bone in the middle ear known an the hammer
(malleus). From the head of the hammer the vibrations pass to the anvil
(incus) and then to the stirrup (stapes). The "sole plate" of this last
bone fits across the small aperture in the wall of the bony
labyrinth known as the oval window
(fenestra vestibuli). The vibrations of the stapes thus pass through this
aperture, along the fluid in the scala vestibuli of the cochlea
, through the helicotrema and down the scala tympani to be dissipated through
the fenestra tympani.

As the vibrations pass along the perilymph of the scala vestibuli they are
transmitted to the endolymph on the cochlear duct and thus to the basilar
membrane. High-pitched sounds cause resonance of the basilar membrane at
the bottom of the cochlea, whereas sounds of lower pitch cause resonance
in places correspondingly nearer to the helicotrema. The vibrations of a
part of the basilar membrane shake the hair cells in the adjacent parts
of the organ of Corti; this causes
them t emit nerve impulses which pass
along the cochlear division of the auditory
nerve to the brain.

Sound appreciation

The fibers of the cochlear nerve are connected to the cortex of the auditory
center in the superior temporal gyrus. Each fiber of the nerve serves only
a short length of the organ of Corti, and is provided with its own particular
area in the auditory cortex. Consequently a pure note, which causes vibration
of only a few hair cells, is responsible for the excitation of only a small,
though exclusive area of cortex. It is this association of each part of
the organ of Corti with a particular area of the auditory cortex which makes
possible the recognition of sounds of different pitch.

The areas of temporal cortex adjacent to the superior temporal gyrus are
concerned with the memory of sounds and their association with the other
senses and the emotions.

Range of hearing

Human hearing extends from a frequency of about
20 cycles per second (Hz) to 20,000 cycles. Some animals are able to generate
sounds far beyond this range: the chart shows the frequencies generated
by bats, porpoises, and grasshoppers, and for comparison the frequency-production
ranges of birds and dogs. Musical instruments have two kinds of frequency-range:
the range of notes that can be played (shown as a solid line) and the range
of overtones that go to make up the characteristic souns of the instrument
(broken line). The ranges shown are those of the violin, the saxophone family
(from bass to soprano), and the harp. For reference, the note middle C is
marked in yellow.